In wireless communication networks of the latest generation and especially in HSDPA-based (High Speed Downlink Packet Access) wireless communication networks data sent from a base station towards one or more mobile terminals (UEs) may be scheduled for transmission on high-speed channels, such as the HS-DSCH (High Speed Downlink Shared Channel).
Also, a UE in these networks may be in different states in a coverage area serviced by a base station, such as CELL_DCH, CELL_FACH, Enhanced CELL_FACH and other states known to the skilled person.
In the CELL_DCH state, data transmitted on HS-DSCH is grouped into transport blocks (TBs) each of which among others comprises a transmission sequence number (TSN) in the header portion of the TB and the user data in the form of MAC-d PDUs (dedicated Media Access Control Packet Data Units) or MAC-c PDUs (common Media Access Control Packet Data Units) in the payload portion. Using a reordering queue as a buffer for transport blocks received and the TSN from the TB header plus control information sent on a control channel a UE can correctly order the TBs received from the base station and forward them to higher layers as MAC-d or MAC-c PDUs. To make the reordering of the TBs more efficient, each UE has a receiver window having a certain size into which TBs with their TSN are received and a timer (T1 timer) which prevents stalling of the TBs in the reordering queue if some TBs are not correctly received.
After a TB is correctly received, the UE updates a parameter indicating the TSN for the next expected TB.
This reordering mechanism for the TBs received at the UE is based on individual (TSNs) and T1 timers, see for example 3GPP TS 25.321. However, when transmitting data on the HS-DSCH using a common H-RNTI, it is not possible to maintain individual TSNs and T1 timers for individual UEs in the network. A single TSN and T1 timer in the network needs to be used for several UEs.
Additionally, in an HSDPA-based wireless network, the Enhanced CELL_FACH state introduces the reception of data on the HS-DSCH in the CELL_FACH state and therefore potentially higher data rates. The reception of data in the HS-DSCH in CELL_FACH state is similar to the reception of data in the HS-DSCH in CELL_DCH state where some of the differences are stated below.
A user equipment (UE) in the Enhanced CELL_FACH state receives retransmissions on HS-DSCH without sending Hybrid Automatic Repeat reQuest (HARQ) feedback signaling (ACK/NACK). Thus, the transport network does not know whether data has been correctly received or not, but rather retransmissions are decided blindly by the network.
Also, the Enhanced CELL_FACH state offers the possibility to transmit data to UEs using a common HS-DSCH Radio Network Temporary Identifier (H-RNTI). An H-RNTI in an HSDPA network is simply a logical address of a UE in a coverage area of a base station. It is possible that more than one UE can share one common H-RNTI. This is needed for UEs that have entered the coverage area of the base station and in the initial HS-DSCH establishment phase do not have a dedicated H-RNTI assigned yet.
Usually in the Enhanced CELL_FACH state the initial value of the next expected TSN is set to 0 in the UE and the discard window is set to [63-WINDOW_SIZE . . . 63], where WINDOW_SIZE refers to the side of the receiver window. The discard window can be defined as the part of the receiver window which is not accepted for re-ordering of TBs.
Now, as the network is using a common TSN for all UEs using a common H-RNTI, there is a likelihood that the first TSN received by the UE is within the discard window. In this case the UE will discard the received transmission.
One solution to this problem has been proposed in “Solution to reordering issue in Enhacned Cell_FACH”, 3GPP TSG-RAN WG2#57bis, Kobe, Japan, May 7-11, 2007. It consists of assigning a special initial value to the next expected TSN, and initializing the discard window based on the first received TSN. This solution avoids the problem of discarding initial transmission. However, during continuous reception, it is necessary for the UE to update the next expected TSN continuously in order to avoid discarding successive transmissions. This requires that all (or several) transmitted TSNs are received correctly by the UE. In practice the HS-SCCH is power controlled and it may be difficult for the UE to receive all TSNs.
The present invention offers a solution to at least some of the problems associated with known technology.